Traditionally, inorganic silicon and gallium arsenide semiconductors, silicon dioxide insulators, and metals such as aluminum and copper have dominated the semiconductor industry. In recent years, however, there has been an increasing research effort in using organic thin-film transistors (OTFTs) as an alternative to the traditional thin-film transistors based on inorganic materials.
Pentacene, thiophene oligomers, and regioregular polythiophenes have been the most widely researched organic semiconductors. Of these classes of semiconducting organic materials, the highest charge-carrier mobility values have been observed for pentacene. Charge-carrier mobility values greater than 1.5 cm2 V−1 s−1, on/off current ratios greater than 108, and sub-threshold voltages of less than 1.6 V have been reported for pentacene-based transistors. These values are comparable or superior to those of amorphous silicon-based devices.
However, the performance of pentacene-based devices can be difficult to reproduce. This lack of reproducibility is due to the polymorphic nature of pentacene. The alignment or structural order of the pentacene molecules differs for each polymorph or crystallographic phase, and this structural order determines the electronic properties of the device. The crystallographic phase adopted by pentacene depends on the process and conditions under which the crystals are formed. For example, when pentacene is vapor-deposited onto a substrate, a thin film phase is formed. This thin film phase is more effective at transporting charge than pentacene's bulk or single crystal phase, but it is meta-stable. For example, the thin film form of pentacene can be converted to the bulk phase by exposure to solvents such as isopropanol, acetone or ethanol. (See, for example, Gundlach et al., Applied Physics Letters, 74(22) 3302 (2000).)
In order to achieve maximum performance, pentacene must generally be deposited from the vapor phase by vacuum sublimation. The vacuum sublimation process, however, requires expensive equipment and lengthy pump-down cycles. Solution processing has the potential to greatly reduce the manufacturing costs associated with the use of organic semiconductors. Pentacene, however, is insoluble in common solvents and is therefore not a good candidate for solution processing.